1. Field of the Invention
The present invention relates to a signal reproducing circuitry in an optical disc player and signal reproducing method thereof . More specifically, the present invention relates to circuitry and a method for reproducing video signals from a laser vision type optical disc on which video signals are recorded, without attenuating high frequency components of the video signals.
2. Description of the Background Art
An optical disc has been known as a medium capable of recording information in a high density. Video and/or audio information is recorded on the optical disc in the manner of presence/absence of reflection of light. The presence/absence of the light reflected therefrom is provided by projections called pits formed on the optical disc corresponding to signals to be recorded.
A system for reading the information recorded on the optical disc by means of a laser beam is generically called an optical disc system. An optical disc on which video information is recorded is called a video disc. An optical disc on which only audio information is recorded is called a digital audio disc DAD. A compact digital audio disc which has the diameter of 12 cm and the thickness of 1.2 mm is called a compact disc CD.
A schematic structure of an optical pickup portion of the optical disc system is shown in FIG. 1. Referring to FIG. 1, the optical pickup portion comprises: a laser light source 1 emitting a laser beam of a prescribed wavelength; a collimator lens 2 and a refractive grating 3 for transporting the laser beam from the laser light source 1 into a parallel beam; a polarized beam splitter (half mirror) 4 for changing the direction of the given laser beam (from the laser light source 1 or from the optical disc 10); a quarter wave plate 6 for delaying the laser beam from the polarized beam splitter 4 and the laser beam reflected from the optical disc 10 by prescribed phases, respectively; an objective lens 7 for decreasing the beam diameter of the laser beam from the quarter wave plate 6 to apply a beam spot having a prescribed diameter on a recording surface of the optical disc and for transforming the laser beam reflected from the optical disc 10 into a parallel laser beam to be applied to the quarter wave plate 6; and a lens 8 for decreasing the diameter of the laser beam reflected from the polarized beam splitter 4 to form a beam spot on a photo detector 9.
A semiconductor laser (laser diode) having a wavelength of, for example, 780nm is employed as the laser light source 1.
The polarized beam splitter 4 allows passage of a laser beam having a particular plane of vibration and reflects a laser beam whose phase is shifted by 90.degree. from the particular plane of vibration.
The quarter wave plate 6 provides a 90.degree. phase delay for a laser beam which goes and returns therethrough. More specifically, the direction of polarization (plane of vibration) of the laser beam from the polarized beam splitter 4 to the quarter wave plate 6 is shifted by 90.degree. from the direction of polarization of the laser beam from the quarter wave plate 6 to the polarized beam splitter 4.
The objective lens 7 applies a beam spot having the beam diameter of about 1.5 .mu.m on the recording surface of the optical disc 10.
The lens 8 comprising, for example, a condenser lens and a cylindrical lens is positioned such that it applies a circular beam spot on the photo detector 9 when the objective lens 7 and the recording surface of the optical disc 10 is spaced apart by a prescribed distance.
The photo detector 9 has for example a quadrant structure as shown in the figure and comprises a pair of photo detectors (for example photo diode) provided in a preceding side and a pair of photo detectors provided in a succeeding side, respectively, with respect to the direction of rotation of the optical disc 10 (the proceeding direction of the pit 11).
FIG. 2 schematically shows a cross sectional structure of the optical disc 10. Referring to FIG. 2, the optical disc 10 comprises a disc base 14 which is a transparent plastic; a reflective layer 13 formed of, for example aluminum on one surface of the disc base 14; and a coating layer 12 for protecting the reflective layer 13. Pits 11 are formed on one surface of the disc base. The width of the pit 11 is generally 0.4 .mu.m. The length and the space between the pits are various ranging from the minimum 0.5 .mu.m to the maximum 2.4 .mu.m dependent on the recorded information. The depth d of the pit 11 is about 0.1 .mu.m, which is selected to be approximately 1/4 of the wavelength of the incidental laser beam so as to ensure presence/absence of the reflection of the beam. The pits are spirally formed on one surface of the optical disc 10 with the track pitch of 1.67 .mu.m. The operation of the optical pickup portion will be briefly described in the following with reference to FIGS. 1 and 2.
The laser beam from the laser light source 1 passes through the collimator lens 2 and the diffractive grating 3 to be a parallel beam. The parallel beam, whose direction is changed by the polarized beam splitter 4, passes through the quarter wave plate 6 to enter the objective lens 7. The laser beam applied to the objective lens 7 is converged to irradiate the reflective layer 13 of the optical disc 10. The laser beam reflected from the optical disc 10 again passes through the objective lens 7 and the quarter wave plate 6 to enter the polarized beam splitter 4. The phase of the laser beam which went and returned through the quarter wave plate 6 is delayed by 90.degree. from the phase of the original laser beam, and the plane of vibration (direction of polarization) is rotated by 90.degree.. Therefore, the laser beam reflected from the optical disc 10 which has passed the quarter wave plate 6 has its proceeding direction changed by 90.degree. by the polarized beam splitter 4 to be converged on the photo dctector 9 through the lens 8. The photo detector 9 converts the given laser beam into electrical signals, from which the information recorded on the optical disc 10 (presence/absence of the pits) is reproduced. When the laser beam irradiates the pit 11, the intensity of the reflected light returning to the objective lens 7 is reduced due to an interference (diffraction effect) of the light reflected from the pit portion (projecting portion) and from the land portion (flat portion). Meanwhile, if there is no pit 11, the reflected light fully returns to the objective lens 7. This difference of the light intensity is detected by the photo detector 9 and is converted into an electrical signal.
It is also possible in the pickup structure to remove the above described quarter wave plate 6 and polarized beam splitter 4 and, instead, to dispose only a half mirror to permit the laser beam traveling from the laser light source onto the disc and to permit only a reflected laser beam reaching the photo detector 9.
If the optical disc 10 is an optical video disc, then both video and audio signals are recorded thereon. A method for recording signals on the video disc will be briefly described in the following.
FIG. 3 shows a schematic structure of a recording signal processing portion. Referring to FIG. 3, the recording signal processing portion comprises an FM modulator 20 for FM modulating analog video signal, FM modulators 21 and 22 for FM modulating 2 channels of analog audio signals, an adder 23 for adding the outputs from the FM modulators 20, 21 and 22, and a limiter 4 for limiting amplitude of the output from the adder 23.
FIG. 4 shows waveforms of output signals from respective portions of the circuits shown in FIG. 3. The operation of recording signals on the optical video disc will be described in the following with reference to FIGS. 3 and 4. An FM wave 1 of the video signal FM modulated by L the FM modulator 20 and FM waves of the 2 channels of audio signals FM modulated by the FM modulators 21 and 22 are added by the adder 23. In the adder 23, the video FM wave 1 is used as a carrier, and the carrier is amplitude-modulated by the audio FM wave 2 to provide a signal 3. The limiter 24 limits the amplitude of the signal 3 from the adder 23 by slicing the same at a prescribed level to provide a recording signal 4. As is clear in FIG. 4, the recording signal 4 is provided by pulse width modulating the video signal carrier by the audio signal. Pits are formed on the recording surface of the optical disc corresponding to "1" and "0" of the recording signal 4.
FIG. 5 shows a frequency spectrum of the recording signal of the optical video disc. In FIG. 5, a frequency spectrum of a recording signal which is a color signal of the NTSC (National Television System Committee) system is shown. A video signal band occupies 4 MHz to 13.5 MHz. The color video signal is so FM modulated that the band becomes 4.2 MHz, a sync tip becomes 7.6 MHz, a pedestal level becomes 8.1 MHz and a white peak becomes 9.3 MHz. The frequency band from 4 MHz to 13.5 MHz include FM deviation range and upper and lower chroma side bands as FM video signals.
The analog audio 2 channels are FM modulated to have the carrier frequency of 2.3 MHz and 2.8 MHz, respectively, with the frequency deviation +100 KHz. In the frequency range lower than 2 MHz of FM audio signal, digital audio signals having the same format as the compact disc CD are recorded by frequency multiplexing. The recording level of the audio signals is lowered by -26dB to -30dB from that of the video signals in order to prevent disturbance of the video signals. The digital audio signals are EFM (eight to fourteen modulation) modulated. The term EFM means a method of converting 8 bit data into 14 bit data, in which 2.sup.8 patterns having the minimum pulse width of 3T, and the maximum pulse width of 11T are extracted from 2.sup.14 patterns, and input 8 bit data are allotted to the extracted 14 bit patterns in one to one correspondence. The reference character T represents a pulse width (transmission rate) of 1 bit data. The operation of reproducing signals from the photo detector 9 shown in FIG. 1 will be hereinafter described.
FIG. 6 shows a conventional circuitry for reproducing signals from the optical disc. The circuit of FIG. 6 comprises three adders 32, 33 and 34.
The photo detector 9 has a quadrant sensor structure including a pair of photoelectric converting elements (for example, photo diode) D1 and D2 in the preceding side and a pair of photoelectric converting elements D3 and D4 in the succeeding side with respect to the direction of the pits.
The adder 32 adds electric signals S1 and S2 from the photoelectric converting elements D1 and D2. The adder 33 adds the electric signals S3 and S4 from the photoelectric converting elements D3 and D4. The adder 34 adds output signals from the adders 32 and 33 to provide a reproduced FM signal (RF signal). More specifically, by adding all the outputs from four photoelectric converting elements D1 to D4, the reflected light image from the optical disc 10 applied to the photo detector 9 is converted into an electric signal, and the signals reproduced by respective photoelectric converting elements D1 to D4 are all added to provide correct reproduced signal corresponding to the recorded information.
A method for controlling rotation of the optical disc may be a CLV (Constant Linear Velocity) method in which density of information can be made constant from the inner track to the outer track of the optical disc or CAV (Constant Angular Velocity) method in which the number of rotation per unit time of the optical disc is made constant may be employed. In the CVL method, the number of rotation of the optical disc per unit time is so controlled that the linear velocity of reproducing points of the optical disc is kept constant. In the CAV method, the linear velocity of the reproducing points changes, so that the length of the pit corresponding to the same data becomes different at the inner track and the outer track of the optical disc.
A frequency characteristics of the pit recorded on the optical disc reproduced by the optic pickup will be represented as ##EQU1## where .lambda. represents wavelength of the laser beam, v represents linear velocity of the optical disc at the reproducing point and NA represents numerical aperture of the objective lens.
As is apparent from the above equation, the output characteristic of the pickup is degraded as the frequency of the reproduced signal becomes higher. The degradation of the frequency characteristics in the higher range of the reproduced signal will be discussed in the following.
A diameter A of a focused beam spot (beam waist) of a common laser beam employed in a pickup for reproducing laser vision is provided in accordance with the following equation from the wavelength .lambda. of the laser beam from the laser light source and from the numerical aperture NA of the objective lens. EQU beam spot diameter=K.lambda./NA,
where K is a constant determined by intensity distribution of the incidental laser beam and by the shape of the lens aperture. When .lambda.=780nm, NA=0.53 and K=1, the beam spot diameter A will be ##EQU2##
The distribution of the light intensity in the beam spot is not actually uniform but varies at the central portion and the peripheral portion of the beam spot as shown in FIG. 7. The beam becomes weak at the peripheral portions. Assuming that a beam spot, having a radius of 0.5 .mu.m at which the intensity thereof decreases by 3dB from that at the central portion, irradiates a recording surface of an optical disc and that the intensity in the beam spot of the radius of 0.5 .mu.m (diameter of 1.0 .mu.m) is uniform. The critical frequency, at which signal reproduction becomes impossible, is found in the following manner, when the recorded information on the optical disc is to be reproduced by the beam spot having the diameter of 1.0 .mu.m.
FIGS. 8A to 8C show positional relation between the beam spot and the pit. In FIGS. 8A to 8C, the reference characters B1, B2, B3 and B4 represent corresponding divided regions of the photoelectric converting elements D1, D2, D3 and D4 on the beam spot. Referring to FIG. 8A, the beam spot 19 illuminates one pit 11 only, and the recorded information is surely reproduced in this case. In FIG. 8C, the beam spot 19 illuminates two spots and the two pits cannot be distinguished from each other, so that the recorded information cannot be reproduced. In FIG. 8B, the photoelectric converting elements (B3, B4) in the succeeding side alone detect the pit 11, which state defines the critical frequency enabling reproduction. Therefore, the critical frequency for reproduction f.sub.max can be provided when we consider a case in which the minimum pit lengths becomes equal to a half of the beam spot diameter L, that is, the radius of the beam spot. In other words, the frequency when the diameter L of the beam spot 19 corresponds to the minimum 1 wavelength of the pulse signals to be recorded can be regarded as the critical frequency for reproduction f.sub.max. Therefore, EQU f.sub.max =V /L
can be applied. The reference character v is a linear velocity at the reproducing point of the optical disc.
For example, in the optical disc player employing the CLV method, the linear velocity of the optical disc is constantly 10.7 m/s, so that ##EQU3##
In the optical disc player employing the CAV method, the linear velocity at the reproducing points of the optical disc changes from 10.7m/s to 32m/s from the inner track to the outer track of the optical disc in correspondence with the reproducing points in the radial direction (with the number of rotation being 1800rpm), and therefore, ##EQU4##
Therefore, when the beam spot diameter is 1.0 .mu.m and the linear velocity of the reproducing points on the optical disc is 10.7m/s, the frequency characteristic of the reproduced signal has a 0 point at 10.2MHz as is shown by a solid line in FIG. 9. The 0 point changes dependent on the linear velocity of the optical disc, so that the higher frequency components of the reproduced signals are modulated by the linear velocity of the reproducing points on the optical disc.
The frequency providing the 0 point is included in the frequency range employed for the presently used laser vision of the NTSC system (the range represented by the character Q in FIG. 9), so that the higher frequency components of the reproduced signals cannot be correctly reproduced.
Meanwhile, in a magazine "TELEVI GIJUTSU" Jan., 1987, pp. 100, the structure shown in FIG. 10 is proposed. As is apparent from a comparison between the structure of FIG. 10 with that of FIG. 1, a delay circuit 37 having a prescribed delay time is interposed between the adders 32 and 34 in the proposed system. The delay circuit 37 delays reproduced signals from the photoelectric converting elements D1 and D2 in the preceding side by a prescribed time period.
Generally, the pit image of the optical disc formed on the photo detector 9 moves at a high speed as the optical disc rotates. Therefore, at the peak of the laser beam reflected from the optical disc, there will be an "offset" between the photoelectric converting elements D1. and D2 in the preceding side and the photoelectric converting elements D3 and D4 in the succeeding side. The time difference (positional offset) in the output waveforms from the photoelectric converting elements D1 and D2 in the preceding side and from the photoelectric converting elements D3 and D4 in the succeeding side are as small as about 10 to 17nsec. However, by simply adding the output from the photoelectric converting elements D1 to D4, the reproduced signal level will be smaller by the above mentioned time difference (phase difference). In order to eliminate the time difference (phase difference), the delay time of the delay circuit 37 is fixedly set at the above mentioned time difference, so as to eliminate the time difference between the output from the photoelectric converting elements D1 and D2 in the preceding side and the output from the photoelectric converting elements D3 and D4 in the succeeding side.
The above described frequency characteristics might be improved by applying the proposed system. More specifically, when the delay time of the delay circuit 37 is set at (L/2.multidot.v) (=a half of the beam spot diameter/linear velocity of the optical disc) in advance, the positional relation between the beam spot 19 and the pit 11 with respect to the electrical signal processing will be as shown in FIG. 11. Namely, the portion of the beam spot detected by the photoelectric converting elements D1 and D2 in the preceding side overlaps with the portion of the beam spot detected by the photoelectric converting elements D3 and D4 in the succeeding side, as viewed from the point of electrical signal processing, so that the diameter L of the beam spot will be equivalently L/2.
Consequently, the frequency characteristics of the reproduced signals will be the curve shown by the dotted line in FIG. 9. Namely, the above mentioned 0 point can be moved from 10.7MHz to 10.7.times.2=21.4MHz. Accordingly, the 0 point goes out of the frequency range Q employed for the present laser vision system, thereby improving the frequency characteristic of the reproduced signals.
However, even if the diameter of the beam spot is made 1/2 equivalently by applying the above proposed system, there still remain the following problem.
In the above described structure, the outputs of the photoelectric converting elements D1 and D2 in the preceding side are delayed by a fixed time period, and the fixed time period is selected to be (L/2)/v. Therefore, due to the change of the linear velocity of the optical disc employing the CAV method or the CLV or due to the change of the linear velocity of the optical disc derived from a jitter of the servo system maintaining the linear velocity of the optical disc at a prescribed value, the above described 0 point inevitably moves and it cannot be fixed. In other words, the high frequency components of the laser vision is modulated by the linear velocity of the optical disc, preventing accurate reproduction of the high frequency components of the reproduced signals.
A focus servo system (not shown) is provided for controlling the position of the objective lens 7 such that a distance between the objective lens 7 and the recording surface of the optical disc 10 is always kept constant. When a jitter is generated in the focus servo system, the focusing position of the laser beam fluctuates and the intensity distribution in the laser beam changes, so that the effective beam spot diameter L (in which the intensity distribution is uniform) changes. However, the delay time cannot be changed corresponding to the change of the effective beam spot diameter, since the delay time of the delay circuit 37 is fixed. Consequently, the aforementioned 0 point moves.
Even if the above mentioned 0 point can be set at the frequency of about 21.4 MHz, the degradation of the high frequency characteristic of the reproduced signals is inevitable, and therefore extensive defining of the reproduced images, which has been strongly desired recently, cannot be realized. The degradation of the high frequency characteristic will be discussed in the following.
In order to extensively define the reproduced images, much information must be recorded. In order to extensively define the reproduced images with the number of rotation of the optical disc kept as it is, that is, the central frequency kept as it is, the frequency range employed for the video signals must be widened. There are three methods of widening the video signal band, which are shown in FIGS. 12B to 12D, respectively. The frequency spectrum of the recording signals of a conventional laser vision optical disc is schematically shown in FIG. 12A for comparison.
In the method shown in FIG. 12B, FM audio signals are eliminated and the FM video signal band is widened in both lower and higher ranges.
In the method shown in FIG. 12C, the FM audio signals are maintained in the similar manner as in the conventional method (FIG. 12A) and only the higher range of the FM video signal is widened. The energy (recording level) of the widened region is doubled.
In the method shown in FIG. 12D, the FM audio signals are eliminated and only the lower range of the FM video signals is widened. The energy (recording level) of the widened lower region is doubled.
However, in the method shown in FIGS. 12C and 12D, the signal processing in the RF stage (after FM modulation) in recording on the optical disc becomes complicated compared with the method shown in FIG. 12B, and it is technically impossible to provide a filter satisfying the frequency characteristics such as shown in FIGS. 12C and 12D. Accordingly, it is the best to realize extensive definition of the reproduced images by applying the method shown in FIG. 12B.
When signals are to be optically reproduced by using an optical pickup as described in the foregoing, the diameter of the laser beam is selected to be wider than the width of the pit in order to surely detect the presence/absence of the pit. Consequently, the resolution in signal detection (pit detection) is decreased, and the higher frequency components of the reproduced FM signals (RF signals) are degraded as shown in FIG. 13. The tendency of degradation becomes more apparent as the frequency of the reproduced FM signals becomes higher and the reproducing position becomes nearer to the disc inner track, in an optical disc employing the CAV method as shown in FIG. 13. The higher frequency components are more degraded in the inner track of the CAV disc for the following reason. As described in the foregoing, the linear velocity at the reproduction points of the CAV disk changes from 10.75m/s to 32m/s from the inner track to the outer track. In order to make the signal reproducing velocity constant from the inner track to the outer track of the optical disc, the length of the pits representing the same signal is made longer at the outer peripheral portions of the disc, and the length of the pit at the outer most track of the disc is three times as long as that of the inner most track. In the case of an optical disc employing the CLV method, the same degradation of the higher frequency as generated in the inner track of the optical disc of the CAV method occurs over all portions of the optical disc.
One of the conventional method to solve the problem of the degradation of the higher frequency characteristics is disclosed in Japanese Patent Laying-Open Gazette No. 80603/1986, in which the higher frequency components of the reproduced FM signals are enhanced. A schematic circuit structure for carrying out the conventional method of enhancing the higher frequency components is shown in FIG. 14. Referring to FIG. 14, the conventional signal reproducing circuit comprises a pickup 41 for optically detecting signals recorded on the optical disc 10 and for converting the same into electric signals; a preamplifier 42 for amplifying outputs from the pickup 41; a high frequency amplifying correcting circuit 43 for amplifying high frequency components of the signals from the preamplifier 42 and for correcting the high frequency components; a limiter 44 for limiting amplitude of the outputs from the high frequency amplifying correcting circuit 43; and an FM demodulator 45 for reproducing video signals by FM detecting the outputs from the limiter 44.
The high frequency amplifying correcting circuit 43 has high frequency correcting characteristics such as shown in FIG. 15. In FIG. 15, the solid line represents an output characteristic of the pickup 41, a chain dotted line represents a signal characteristic after the high frequency correction, and the dotted line represents correction characteristic of the high frequency amplifying correcting circuit 43. The reference characters A B and C represents disc outer periphery (diameter 290 mm), the disc central portion (diameter 200 mm) and the disc inner track (diameter 110 mm), respectively. As shown in FIG. 15, the amount of correction of the high frequency amplifying correcting circuit 43 becomes the largest at the inner track of the disc.
In reproducing information on the optical disc employing the CAV method, the amount of correction of the high frequency amplifying correcting circuit 43 is switched in correspondence with the reproducing position in the radial direction of the optical disc to correct high frequency components of the reproduced FM signals. Actually, in an optical disc employing the CAV method, the high frequency correction is carried out only in reproducing the inner track, and the correction is not carried out in reproducing the central and outer peripheral portions. In an optical disc employing the CLV method, the same high frequency correction as carried out in reproducing inner track of the CAV disc is carried out for all the portions of the disc.
The selection of the amount of high frequency correction by the high frequency amplifying correcting circuit 43 is generally carried out in the following manner. A plurality of high frequency correcting amounts are stored in advance in an ROM (Read Only Memory), for example. The ROM stores the high frequency correcting amounts in the form of a table using the reproducing positions of the pickup 41 as addresses. The reproducing positions of the pickup 41 are detected by, for example, a position transducer to be converted into digital signals. In accordance with the digital converted information indicative of the reproducing position, the high frequency correcting amount is read from the ROM and the high frequency amplification of the reproduced signal is carried out by the read out amount of correction. The amount of high frequency correction is merely switched stepwise in accordance with the detected data of the reproducing position as shown by the dotted lines A, B in FIG. 15, for example. Therefore, it is very difficult to carry out optimal high frequency correction by delicately changing the amount of high frequency correction corresponding to the ever changing reproducing positions. In order to finely change the amount of high frequency correction corresponding to the reproducing positions, the capacity of the ROM storing the amounts of high frequency correction in the form of a table must be increased, which is not desirable from the point of cost.
Therefore, when the signal recording band is extended into the high frequency range and the influence of the high frequency degradation becomes apparent as in the case of the video disc record realizing extensive definition of images, the conventional high frequency correcting method is not effective.